I’ve just moved into a house that has a hot tub (lucky me!). It uses a lot of power to keep it warm so I’d like to try heating it from the sun. Although a solar water might be more normal, I’d like to try heating it with a solar PV system. I live in Berkeley, CA and the google calculator thing tells me I’ll get 1756 hours of usable Sun. I can get used panels locally for $0.60/W. So my idea is to connect a panel directly to hot water heating element and put that in my hot tub. At this stage I’m looking for some validation on the overall idea, my particular calculations and any safety tips.

Vmp|Imp|Pmax|Voc|Isc|Ideal resistance

Solar panel 1|35.68|8.15|290.792|44.9|8.94|4.37791411|

Using the Vmp and Imp I calculated the “ideal resistance”. I think it’s vital that the resistance of the heating element matches this pretty closely. I found a 600W 48V element which should have a resistance of 3.84 ohms which is pretty close (and I think gives me an efficiency of 87% based on (8.15 * 3.84 * 3.84) to get the power and dividing that by Pmax).
I can do slightly better by getting a pair of cheaper and smaller mains heating elements 120V 1650W which have a resistance of 8.72 ohms or 4.36 for two in parallel. Which is now very close to my ideal!

Question1: For this ideal resistance calculation, I’m assuming the panel will be operating at Pmax, but presumably the panel never will be (unless it’s directly facing the sun on a clear day). Should I be calculating a different ideal resistance based on more realistic typical conditions?

Question2: What fuses and isolation switches should I have? I plan to have an isolation switch between the panel and the heating element (is something like this good enough). Do I need any fuses too? I can’t really get my head round how a fuse could help. I’m basically trying to short the panel out in the most efficient way possible, so a fuse will always be sized for more current than the panel could ever provide!

Question3: Any tips on cabling that’s OK if it comes into contact with the hot tub water (which contains some chlorine).

\$\begingroup\$You think you can do this with ONE panel? Show us your calculation of the number you think will do the job.\$\endgroup\$
– FinbarrApr 7 '18 at 17:12

7

\$\begingroup\$The only way you'll get power currant is by raisin the voltage.\$\endgroup\$
– TransistorApr 7 '18 at 17:15

\$\begingroup\$Vmp and Imp are typically given at 25C. Most panels also list the Voltage and Current that produce maximum power at 40C, which is more realistic, because the panels warm up when they are in the sun. Use those values to calculate your ideal resistor value.\$\endgroup\$
– mkeithApr 7 '18 at 17:39

1

\$\begingroup\$Why don't you just add around 2 kW of grid tie solar panels. Put a timer on the hot tub heater so it only runs during the day.\$\endgroup\$
– mkeithApr 7 '18 at 20:03

\$\begingroup\$There is a reason that the ocean is not heated with electricity.\$\endgroup\$
– user56384Apr 8 '18 at 1:05

3 Answers
3

Sadly the big flaw is that you want hot tubs in winter, when you have no spare solar power.

I have vacuum tubes to heat water (40 tubes for 4 people). In summer these have 5kw excess heat from 11am onwards - but no desire for a hot tub. In winter it can only just heat the water tank, and the PV panels make average 50W per 500W capacity. (PV panels are really poor under weak sun).

Good flat water heating panels from China would be far and away the best solar choice, but realistically, a heat pump is better yet.

For the DIY enthusiast, you can carefully rip an old fridge apart without breaking the sealed piping, and turn it into a small heat pump. My one can pump about 9kWh/day. Old fridges are free (often just the auto defrost controller fails). Catch: you do get a god awful big pile of foam bits and plastic left over

I apologize. I did not read everything. It is really stupid to use photovoltaics (PV) to heat water. The reason is that you will, at best, convert 20% of incident sunlight into heat.

But a solar hot water heater may approach 80% or 90% efficiency, even for crude do-it-yourself (DIY) panels.

My suggestion is to build a simple hot water panel using black irrigation tubing. Coil several hundred feet of tubing on a piece of plywood. Put a pane of glass over it if you like. Keep that in the sun. Get one small PV panel to run a small 12V pump that pumps water through the tubing when the sun shines. This will probably not heat the hot tub all the way up to usable temperature unless you put a bunch of them in series. But it will make it will raise the baseline temperature noticeably.

Maybe a bait circulation pump or something like that will do the job. You don't need a battery. There is no point in running the pump when the sun is not shining.

Thanks to peufeu for pointing out that hot tubs (spas) are a potential risk area for Legionnaires' disease. This type of heating could possibly raise the risk a bit because the water may never be heated hot enough for long enough to kill the bacteria responsible. Conventional mains powered electric heating may be hot enough to flash kill the bacteria. In either case, keeping the water chlorinated is advised to minimize risk.

\$\begingroup\$This is a really good idea. Use the sun to heat a large thermal mass (the water tank) and then the coil and pump circulating through it back to the hot tub makes a great heat exchanger.\$\endgroup\$
– Captain BarnaclesApr 7 '18 at 17:40

\$\begingroup\$Legionella bacteria can develop in warm water so if you use thermal solar panels to heat a large volume of water, you need a heat exchanger to keep the bacteria out of the bath water. Another solution is to use solar to pre-heat water, which then goes into the boiler (legionella dies at 60°C). But please be aware that water between 30-40°C is an ideal breeding ground for this pathogen. It's not difficult to avoid problems, just need to be aware of it.\$\endgroup\$
– peufeuApr 7 '18 at 19:42

\$\begingroup\$@peufeu, it is a hot tub. The hot tub will spend a lot of time at a perfect temperature for legionella regardless of heating system. The easiest solution is to just keep it chlorinated. What I propose is not to use a heat exchanger, but to just pump the (presumably chlorinated) water through the tubing. Chlorine kills legionella very effectively.\$\endgroup\$
– mkeithApr 7 '18 at 19:56

1

\$\begingroup\$Yes chlorine is a good solution too. I was insisting about it just in case OP wasn't aware.\$\endgroup\$
– peufeuApr 7 '18 at 20:17

For this ideal resistance calculation, I’m assuming the panel will be operating at Pmax, but presumably the panel never will be (unless it’s directly facing the sun on a clear day). Should I be calculating a different ideal resistance based on more realistic typical conditions?

Unless you use a maximum power point tracker, you will get less than X% nominal power when the irradiance is X% of nominal. Since you’re just trying to create heat, a shunt regulator would be acceptable.

Question2: What fuses and isolation switches should I have? I plan to have an isolation switch between the panel and the heating element (is something like this good enough). Do I need any fuses too? I can’t really get my head round how a fuse could help. I’m basically trying to short the panel out in the most efficient way possible, so a fuse will always be sized for more current than the panel could ever provide!

The purpose of fuses is to prevent a fire in the event of a short circuit. If your system does not provide enough power to start a fire at a short, they serve no purpose.

Question3: Any tips on cabling that’s OK if it comes into contact with the hot tub water (which contains some chlorine).

If the resistance wire is in good thermal contact with the water it cannot get much hotter than 100C. So you can use insulated wire.

I fully agree with the other answer that this is not an efficient solution and you should heat the water directly with sunlight instead.